In recent years, with the increasing reduction in size and the advance in functionality of electronic devices, such as mobile phones, PDAs (Personal Digital Assistants), laptop PCs and the like, high-density packaging of electronic components used in them is becoming essential. Conventionally, high-density packaging of electronic components has been achieved by making component terminals more fine-pitched as a result of making electronic components smaller, by making the conductor pattern on a printed circuit board on which electronic components are mounted finer, and so forth.
In addition, in recent years, the development of multi-layer printed circuit boards which make three-dimensional wiring possible by layering printed circuit boards is being advanced, and further, the development of device-incorporated substrates that aim to further improve packaging efficiency by having electronic components, such as chip resistors, chip capacitors and the like, and electric devices, such as semiconductor chips and the like, built into these multilayer printed circuit boards is also being advanced.
As a method of forming a conductor pattern for a printed circuit board, a transfer method using a transfer sheet is conventionally known. The process of manufacturing a printed circuit board using this transfer method includes, mainly, a pattern formation step for forming a conductor pattern on one surface of a transfer sheet, and a pattern transfer step for removing the transfer sheet after the transfer sheet has been adhered to an insulation layer with the formed conductor pattern in between.
A printed circuit board produced through a transfer method can be easily multilayered by forming, in desired places of the insulation layer, via-holes for connecting the layers.
As conventional art of this sort, for example in Japanese Patent No. 3051700, a method of manufacturing a device-incorporated substrate using a transfer method is disclosed. Hereinafter, a conventional method of manufacturing a device-incorporated substrate will be described with reference to FIG. 11A through FIG. 11F.
FIG. 11A through FIG. 11F are stepwise sectional views showing a conventional method of manufacturing a device-incorporated substrate. A void section 32 for housing a semiconductor chip 36 and via-hole conductors 33 for connecting layers and which are made by filling through-holes with a conductive paste are each formed in an insulating base material 31 (FIG. 11A). On the other hand, on one side of a transfer sheet 34 is formed a conductor pattern 35 to be transferred onto the insulating base material 31 (FIG. 11B).
Here, the insulating base material 31 is comprised of a thermo-setting resin that is partially cured, and the transfer sheet 34 is comprised of a resin film of polyethylene terephthalate (PET) or the like. In addition, the conductor pattern 35 is formed by performing pattern etching on a conductor foil, such as a copper foil or the like, that is adhered to the transfer sheet 34 in advance.
Next, the semiconductor chip 36 is bonded to a predetermined position of the conductor pattern 35 formed on the transfer sheet 34 (FIG. 11C). Then, the upper surface of the insulating base material 31 and the side of the transfer sheet 34 on which there is the conductor pattern 35 are pressure bonded and the semiconductor chip 36 is housed inside the void section 32, while the conductor pattern 35 is connected with the via-hole conductors 33 (FIG. 11D). The conductor pattern is buried in the upper surface of the partially cured insulating base material 31, and thereafter, just the transfer sheet 34 is removed from the insulating base material 31. Then, by completely curing the base material 31 through a heat treatment, a device-incorporated substrate 30 is obtained (FIG. 11E).
In addition, as shown in FIG. 11F, by layering insulating base materials 39 and 40 on which conductor patterns 37 and 38, respectively, are formed through a method similar to the one above onto the above-mentioned device-incorporated substrate 30, a multi-layered printed circuit board 41 is obtained.
However, in this conventional method of manufacturing a device incorporated substrate, because the transfer sheet 34 is comprised chiefly of a resin film, there is a problem in that due to stretching and warpage in the transfer sheet 34 caused during handling, errors in the pattern configuration of the conductor pattern 35 to be transferred occur with greater likelihood. Therefore, with this conventional method of manufacturing a device-incorporated substrate, it is extremely difficult to accommodate the trend towards finer (more fine-pitched) conductor patterns, which is to progress further in the future.
In addition, the conductor pattern 35 formed on the transfer sheet 34 is formed, as disclosed in Japanese Patent Application Publication No. HEI 9-270578 for example, by pattern etching a metal foil adhered onto the transfer sheet 34, or, as disclosed in Japanese Patent Application Publication No. HEI 10-335787 for example, by pattern etching a metal layer that is formed on the transfer sheet 34 directly through sputtering or the like. As the method of etching, wet etching is adopted.
In other words, in the conventional method of manufacturing a device-incorporated substrate, because wet etching is used in the formation of the conductor pattern 35, there is a problem which is that, in the future, it is going to become difficult to form fine-pitch patterns with high precision.
On the other hand, it is also conceivable to have the transfer sheet be made of a metal material such as stainless steel or the like. In this case, because the rigidity is higher as compared to a case where the transfer sheet is made of a resin film, the dimensional stability of the conductor pattern is improved. However, in this case, there is a problem in that if the rigidity of the insulating base material, which is the transfer target, is high, it becomes difficult to remove the transfer sheet from the insulating base material and the transfer operation for the conductor pattern cannot be performed properly.
The present invention is made in view of the problems above, and makes it an issue to provide a method for manufacturing a device-incorporated substrate in which the dimensional stability of the conductor pattern is secured to make it possible to form a fine-pitch conductor pattern on an insulating layer with high precision and in which the removal of the transfer sheet can be performed properly.